In integrated circuit production facilities, integrated circuits are fabricated with a process step which deposits impurity ions into the silicon surface by irradiating the silicon wafer with a beam of accelerated dopant ions. To perform this process step correctly, the crystalline lattice of the silicone wafer must be angularly aligned with the ion beam in two orthogonal degrees of freedom. During implant, the wafer is placed on a flat pad (the implant pad). Often, several implant pads are mounted on a large spinning disk so that several wafers may be processed in parallel. The disk can be tilted with respect to the ion beam to control one angular degree of freedom (tilt). To control the second angular degree of freedom (twist) a sensor is used to measure the rotational position of a reference notch (or flat) on the wafer and then a rotational actuator rotates the wafer so that it will be placed on the pad in the correct twist relationship to the ion beam. The process of measuring and actuating for the correct wafer twist is commonly referred to as wafer orientation.
Conventional ion implanters incorporate several mechanical actuators and sensing systems to accomplish the orient operation. In a conventional implanter, an un-oriented wafer is robotically moved to and placed on an orienting pedestal. The pedestal then rotates to enable a sensor to determine the distance from the center of rotation to the perimeter of the wafer. Since the twist reference notch (or flat) on the wafer causes the measured perimeter to decrease, the rotational position where the perimeter is a minimum represents the rotational position of the notch (or flat). This description assumes that the wafer is centered on the pedestal when the orientation is measured. In reality, it is not. An x/y centration error is commonly referred to as an eccentricity error. The data set describes both the centration (eccentricity) and the rotational position of the notch (twist). In a conventional implanter, this data set is analyzed with computer algorithms to determine the wafer rotation and centration on the pedestal.
In a conventional implanter, following the measurement of the wafer centroid and rotation al position, the wafer is rotated to correct the measured rotational error. The wafer is then picked up by a robot and received by a pedestal (wafer lifter) which lowers the wafer to the correct X,Y and rotational position on the implant pad. To accomplish the wafer handling and orientation operation in a conventional implanter system, the wafer is commonly handled by at least four robotic actuators, including:
1) a wafer shuttle which moves the wafer from the loadlock (containing a cassette full of wafers to be processed) to the orient pedestal. PA1 2) an orient pedestal which rotates the wafer so the X,Y and rotational positions can be measured. After establishing these positions, the orient pedestal rotates the wafer to the correct rotational position. PA1 3) an orient to pad robot which moves the wafer from the orient pedestal to a position above the implant pad. PA1 4) a wafer lifter which lowers (raises) the wafer from (to) the robot to (from) the implant pad.